Yes it is the right decision, and no it is not murder. For these reasons:

- Tuvix is one person. Neelix and Tuvok are two. The greatest good for the greatest number.

So you'd be willing to have anyone just kill you and harvest your organs if it would benefit two or more people's lives? Wait, you don't have to be willing. Greatest good for the greatest number.

- Tuvix owes his entire existence to a transporter malfunction. He was not supposed to exist.

Who are you to decide that? Who is Janeway? So only people who are actively given birth to have the right to life? What about cloning, do they have no rights just because of the method of their creation? The only fact that matters is he DOES exist.

- Retroactive though this may be, Tuvok and Neelix would later express gratitude that they were able to be separated. And the mere fact that they were able to be separated proves that they were never 'dead'...

I'm sure they are, but that's irrelevant to the discussion. For all practical purposes they were gone when Tuvix was created. That Janeway found out how to literally cut open a person against his will and give life to two others is the issue. Janeway has ZERO right to decide that she can play god and kill a person for her perceived benefit.